Microwave transmission and reception by phased array antennas is increasingly used for radar, communication and data transmission. This is because phased array antennas have many advantages over older conventional antennas. The phased array antennas make use of an array of fixed individual radiators to produce electronically steered signals that are only decipherable in a preferred direction when the signal arrives with a coherent wavefront. Further, the directionality of the beam lends itself to use with radar detectors in order to pinpoint aircraft position. Such radars are commonly used for air traffic control. The advantages of using phased array antennas may be summarized as permitting pinpoint radar detection without moving antenna systems, providing voice and data communication to a desired receiver rather than in a general broadcast and permitting extremely fast and agile changes in radio beam direction.
The main drawback to increased successful use of phased array antenna systems stems from their use of costly and bulky conventional microwave radio frequency phase shifters. The systems currently used to provide coherently steered signals from arrays of individual radiators are extremely complicated and expensive electronic devices. This is because large numbers of electronic phase shifters (one for each individual radiator) are required to drive the antenna. The driving circuitry of electronic phase shifters is quite complex and requires relatively large amounts of electric power for programmed operation of phased array scanning and beamforming.
An attempt to utilize optical devices in place of conventional phase array control devices is disclosed in U.S. Pat. No. 3,878,520 to Wright et al. The Wright device, if operable, describes a scheme utilizing a bulk optical pattern to vary spatial beam position. An optical pattern is created at an optical to microwave converter (56) by light into apportioned light pipes which are controlled by a light valve control to produce a desired light pattern gated at a microwave frequency. Wrights' free space optical phase processor is shown in FIG. 9 of the subject patent.
While the optic approach of the Wright device may be an advance in the art of phased array signal generation, it unfortunately has considerable disadvantages as a practical device. The Wright device uses one large optical mixer as an optical phase processor. Light is captured in an array of gated light pipes which are selectively controlled to provide an image at a lens. This process is very wasteful of light (only 10-20 percent will probably proceed through the light pipes) and relies on optical beams propagating in air. This can be inaccurate and subject to disruption from dust and vibration. The microwave beams are steered physically by gating the light pipes selectively. Finally, it is unlikely that the Wright device can produce more than a small, limited number of antenna beam positions.
In summary, the Wright device does display some of the advantages of using an optic approach, such as use of fiber optic filaments, however, the complex and sensitive means provided for optical phase processing must be considered a low efficiency and somewhat clumsy mechanism.
In view of the above a need is apparent for an improved, preferably optical, beamsteering device for a phase array antenna.
It is therefore an object of this invention to provide a complete optical beamforming network for generating signals that excite a phased array antenna system to produce a desired directionally controlled microwave beam.
It is further an object of this invention to provide a straight forward and inexpensive electrooptical device which forms an optical microwave phase shifter capable of producing zero to greater than 2.pi. of electrical phase in the microwave output.
It is yet another object of this invention to provide an optical phase shifter for phased array antenna steering that is substantially more compact than conventional structures and is suitable for inclusion in an integrated optical circuit.
It is still another object of this invention to provide a phase shifter structure through which light is completely guided by single mode optical fibers and suitable channel waveguides.
Finally, another object of this invention is to provide for an inexpensive electrooptical arrangement which can be mass produced for use with phased array antennas.